Mixed potential sensors are used for the detection of various oxidizable and reducible gases. Oxidizable gases include hydrogen, hydrocarbons, carbon monoxide, nitric oxide, ethanol, and the like, while reducible gases include oxygen, nitrogen dioxide, and the like. Typical sensors utilize an ionic conducting electrolyte, such as yttria stabilized-zirconia (YSZ), and thin film metal and/or metal oxide electrodes, such as platinum (Pt) and perovskite-type oxides. Multiple reduction/oxidation reactions occurring between the gas phase and the electrode/electrolyte interface cause mixed potentials of differing magnitude to develop at the dissimilar electrodes. The selectivity of such sensors is achieved by the proper selection of the metal and metal oxide electrodes, while the stability of the sensor is achieved by the precise control of the surface area of the electrode and the 3-phase interface region (gas-electrolyte-electrode) of the sensor.
The mixed-potential that is developed at an electrode-electrolyte interface in the presence of oxidizable gases, such as CO or hydrocarbons, is fixed by the rates of reduction and oxidation of the oxygen and the oxidizable gas, respectively (Reactions 1 and 2). Carbon monoxide is used here as an example but the theory is also applicable to other oxidizable/reducible gases:

When the kinetic rate for the reduction reaction equals that of the oxidation reaction a stable potential is established. Similar reactions with different kinetics occur on the other electrode triple-phase boundary area. The difference in potential between the electrodes is the device output voltage. The preferred mixed potential CO sensing device consists of an electrode that kinetically inhibits the oxygen reduction reaction yet is fast at CO electro-oxidation and a second electrode that exhibits fast oxygen reduction kinetics yet is poor at CO electrochemical oxidation.
Since the mixed-potential is controlled by the kinetics of various reactions, control of the 3-phase (electrolyte/electrode/gas) area is of importance. Moreover, since the response time of these sensors is fixed by the speed of various reactions, the sensor design is preferably optimized to maximize the rates of reactions 1 and 2.
In the prior art, a dense YSZ electrolyte is used as the substrate and a metal or metal oxide electrode is deposited on top of this electrolyte. The length of the active 3-phase interface is controlled by the morphology of the electrode. A highly porous electrode results in better gas access and greater 3-phase interface, whereas a denser electrode leads to poorer gas access and less 3-phase interface. One drawback of this type of arrangement is that the gas has to meander through the pores of a catalytically active material (the electrode) before reaching the 3-phase interface where the reduction and oxidation reactions occur. Hence, the hydrocarbons (or other reducing gases) are heterogeneously oxidized at the metal (or metal oxide) electrode before they reach the 3-phase interface with the electrolyte, with a concomitant loss in sensor sensitivity and increase in response time.
Mukundan et al. (U.S. Pat. No. 6,656,336, issued Dec. 2, 2003) disclose a non-methane hydrocarbon sensor that measures the amount of hydrocarbons present in an exhaust stream containing oxygen. The selectivity of the device is achieved by the proper selection of the oxide electrode, while the stability of the device is achieved by the precise control of the surface area (SA) of the electrode and the 3-phase interface region (3 PA) (gas-electrolyte-electrode) of the sensor. By controlling the ratio of the SA to the 3PA, the rates of the heterogeneous catalysis and electrochemical catalysis are controlled for any particular electrode used. Thus, by proper selection of the electrode material and electrode dimensions, the magnitude of sensor response to any particular gas species can be amplified (selectivity).
Mukundan et al. (U.S. Pat. No. 6,605,202, issued Aug. 12, 2003) disclose a mixed-potential electrochemical sensor for the detection of gases, such as CO, NO, and non-methane hydrocarbons, in room air. The sensor utilizes a ceria-based electrolyte, and metal wire electrodes. The stability and reproducibility of the sensor is achieved by using wire electrodes instead of the usual thin or thick film electrodes that are currently employed. The metal wire-electrodes are directly embedded into the electrolyte and co-sintered with the electrolyte in order to produce a stable metal/electrolyte interface.
The present invention uses thin film technology to produce a mixed-potential electrochemical sensor for the detection of gases, such as CO, NO, and non-methane hydrocarbons, where the sensor exhibits fast response time, good reproducibility, and can be produced using inexpensive thin film technology.
Bloemer et al. (U.S. Pat. No. 6,352,631, issued Mar. 5, 2002) teach a mixed-potential sensor formed by depositing electrodes on a dense electrolyte, where the electrodes are exposed directly to the gas stream to be sampled. Muller et al. (U.S. Pat. No. 4,277,323) teach an oxygen sensor where two electrodes are put on a porous substrate and completely covered with a thin YSZ film. While this works well for an O2 sensor and improves performance, this will not provide a mixed potential sensor. Further, Mueller et al. provide gas access only through a porous substrate onto an electrode embedded in an electrolyte where the 3-phase interface region is not well defined.
Various objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.